1. Field of the Invention
This invention generally relates to an optical waveguide device for use in an optical pick-up or the like, and, in particular, to a covered optical waveguide device whose optical waveguide is covered at least partly and an inlet opening for guiding light into the optical waveguide is formed in a cover layer.
2. Description of the Prior Art
In order to have incident light optically coupled to an optical waveguide, use may be made of any of several prior art methods, such as prism coupling, grating coupling and end surface coupling. Among these prior art methods, the prism coupling method is often preferred because of its easiness in application and high efficiency in having incident light coupled into the associated optical waveguide. The prism coupling method is schematically illustrated in FIG. 9 and the principle of this method can, for example, be found on pages 237-243 of "OPTICAL INTEGRATED CIRCUIT" by Hiroshi Nishihara, OHM Publishing Co., Ltd. For completeness, the principle of the prism coupling method will be briefly described with reference to FIG. 9 below.
A substrate 1 having an index of refraction n.sub.s is formed with an optical waveguide 2 at its top, and a prism 3 having an index of refraction n.sub.p is placed in close proximity with the top surface of the optical waveguide 2 with a thin layer of a medium 4, typically air, sandwiched therebetween. Under the condition, when a light beam 5 is directed at an angle .theta. with respect to the bottom surface of the prism 3, the propagation constant B of this light wave in the Z direction shown in FIG. 9 may be expressed by the following equation. EQU B=n.sub.p .multidot.k.multidot.sin .theta. (1)
where, k=2.pi./.lambda. and .lambda.: wavelength of incident light. Denoting the angle of the apex of the prism 3 by .alpha., then, according to the Snell's law, the incident angle .theta.' outside of the prism 3 may be expressed by the following equation. EQU n.sub.c .multidot.sin(.theta.'-.alpha.)=n.sub.p .multidot.sin(.theta.-.alpha.) (2)
Now, if the gap or spacing S between the prism 3 and the optical waveguide 2 is larger, the incident light beam 5 will be totally reflected at the bottom surface of the prism 3 when the angle .theta. satisfies the condition of .beta./k being larger than n.sub.c ; however, evanescent waves are produced in the medium 4 having an index of refraction n.sub.c. Accordingly, if the angle .theta. is adjusted so as to make the value of .beta. shown in equation (1) equal to the propagation constant of a certain guided mode as the spacing S is made gradually smaller, the evanescent wave produced in the optical waveguide 2 comes to be matched in phase with the guided mode, so that a distributed coupling occurs to thereby produce guided light.
Denoting the distribution of amplitude of the guided light and incident light beam 5 in Z direction by EQU g(z)=exp (-.alpha.r.multidot.z),
the coupling efficiency according to this method is predominantly determined by the magnitude relationship between the beam width and 1/.alpha.r. And, if the value of this 1/.alpha.r is in the order to 10 microns, the spacing S will be in the order of submicrons. Besides, since .alpha.r is strongly dependent upon S, an extreme care is required in adjustments of spacing S.
Furthermore, if the incident light beam 5 is directed toward the front end of the bottom surface of the prism 3 rather than toward the rear end as shown in FIG. 9, the light which has once been completely coupled into the optical waveguide 2 again may go out of the optical waveguide 2 in the form of evanescent wave during its propagation along the optical waveguide 2. Thus, in this case, light may be coupled into the optical waveguide 2 through the prism 3 at one point, but the light may also be decoupled from the optical waveguide 2 at another point through the prism 3. As a result, the determination of point of coupling of light into the optical waveguide 2 needs careful consideration and adjustments.